window.alert = function(str)  
{  
    var shield = document.createElement("DIV");  
    shield.id = "shield";  
    shield.style.position = "absolute";  
    shield.style.left = "0px";  
    shield.style.top = "0px";  
    shield.style.width = "100%";  
    shield.style.height = document.body.scrollHeight+"px";  
    //弹出对话框时的背景颜色  
    shield.style.background = "#fff";
    shield.style.opacity="0.5";
    shield.style.textAlign = "center";
    shield.style.zIndex = "25";  
    //背景透明 IE有效  
    shield.style.filter = "alpha(opacity=0.5)";  
    var alertFram = document.createElement("DIV");  
    alertFram.id="alertFram";  
    alertFram.style.position = "absolute";  
    alertFram.style.left = "50%";  
    alertFram.style.top = "50%";  
    alertFram.style.marginLeft = "-225px";  
    alertFram.style.marginTop = "-150px";  
    alertFram.style.width = "450px";  
    alertFram.style.height = "150px";  
    alertFram.style.background = "#ffffff";  
    alertFram.style.textAlign = "center";  
    alertFram.style.lineHeight = "150px";  
    alertFram.style.zIndex = "300";  
    strHtml = "<ul style=\"list-style:none;margin:0px;padding:0px;width:100%;\">\n";  
    strHtml += " <li style=\"background:#333333;text-align:left;padding-left:20px;font-size:14px;font-weight:bold;height:30px;line-height:30px;color:#fff\">提示信息</li>\n";  
    strHtml += " <li style=\"background:#fff;text-align:center;font-size:16px;height:120px;line-height:120px;border-left:1px solid #333333;border-right:1px solid #333333;\">"+str+"</li>\n";  
    strHtml += " <li style=\"background:#333333;text-align:center;font-weight:bold;height:30px;line-height:30px; border:1px solid #F9CADE;\"><input type=\"button\" value=\"确 定\" onclick=\"doOk()\" style=\"width:40px;height:25px;border:none;background:#fff;\"/></li>\n";
    strHtml += "</ul>\n";  
    alertFram.innerHTML = strHtml;  
    document.body.appendChild(alertFram);  
    document.body.appendChild(shield);  
    var ad = setInterval("doAlpha()",5);  
    this.doOk = function(){  
    alertFram.style.display = "none";  
    shield.style.display = "none";  
    }  
    alertFram.focus();  
    document.body.onselectstart = function(){return false;};  
}  